(1) Field of the Invention
The present invention relates to transmission of large amounts of data between a PABX and a switching center via a data link, and more particularly to a circuit for automatically synchronizing the clocking of data received via the data link, thereby insuring the validity of the data.
(2) Description of the Prior Art
Telephone switching centers are connected to PABX's, located on subscriber premises, via trunk circuits. Many individual stations are connected to the PABX. A relatively smaller number of trunk circuits connect the PABX to the switching center. Therefore, each PABX station must dial an access code digit to seize control of an available trunk circuit.
As a result of this operation, the switching center is unable to distinguish the identity of the particular station originating the telephone call. Historically, operators were utilized to ask the particular station user for his station identity before connecting the call. Such information was manually recorded by the operator.
With the advent of electronics, sophisticated systems for the transmission of this station information from the PABX to the central office were developed. This equipment consisted of electronics located on the PABX subscribers premises, data link equipment connecting the subscriber premises to the switching center, and additional electronics added to the switching center. These systems are termed automatic identification of outward dialing systems. Such systems provide the switching center with the identity of the calling station automatically and without the need of operator intervention.
When a PABX station user dials the access code for a trunk circuit connecting him to the switching center (central office trunk), the station identity is noted at the PABX. Also noted is the identity of the trunk circuit selected, connecting the station through the PABX to the switching center. Typically, these two pieces of information are combined into a 41-bit transmission. The station identity consists of 20-bits, the trunk identity 20-bits and a 1-bit synchronization mark. The above information is transmitted via a separate data link facility to the switching center where it is placed in a temporary storage buffer.
As the central office trunk is seized at the PABX, it causes a "Call-For-Service" to be generated at the switching center. When the "Call-For-Service" is recognized, the switching center identifies the requestor by the trunk identity stored in the center's data base. At convenient points in servicing the call, the temporary storage buffer is searched using the trunk identity obtained when the "Call-For-Service" was recognized. Upon finding a trunk identity concurrence, the station identity is placed into the switching centers memory corresponding to the call. Using the above identified trunk, a billing record is generated including the particular station identity.
The collected information is stored on a suitable output device and interpreted by an electronic data processing center. The processing center is able to generate a detailed billing document containing the charges and the number of calls made by each station within the PABX. These detailed billings aid the corporate customer in accounting for its telephone charges.
Generally, the transmission of data to a remote place via a data link is accomplished in one of two ways. First, a synchronous data link can be used to connect two remote locations. Such data links are very expensive and require dedicated communication line and logic at each location. Second, an asynchronous data link may be utilized, but such links require synchronizing signals to be transmitted periodically in order to avoid the loss of data. Typically, asynchronous data links are less expensive then synchronous data links.
The proper reception of data is of particular importance in situations in which telephone subscribers are charged based on this data. In the system of which the present invention is a portion this invention deals with the automatic identification of outward dialing of a PABX subscriber. Information identifing the particular subscriber placing the call and the identity of the trunk circuit used to connect him to the central office is transmitted to the central office for use in the preparation of billing the associated subscriber.
Synchronization circuits are disclosed in U.S. Pat. Nos. 4,095,053 issued to D. L. Duttweiler, et al, on June 13, 1978; 4,045,618 issued to J. Lagarde, et al, on Aug. 30, 1977; and 4,022,845 issued to P. Kaul et al, on Jan. 11, 1977.
In Duttweiler et al, synchronization of data reception is achieved by the stuffing of pulses into the data stream. Blocks of pulses are stuffed into the data bit stream at the transmitting location and must be de-stuffed at the receiving location. Such a system provides a relatively slow rate of acquisition of data. That is, since many pulses are stuffed into the information data stream, the full capacity of the data link is not utilized.
Lagarde et al teaches the use of synchronization of an information data stream by comparison to a known reference information data stream. Such a system requires memory and extensive control logic, thereby rendering this system complex and expensive.
The Kaul patent discloses a frame synchronizer which searches for synchronization bits included in the information transmitted. One drawback of this arrangement is that a synchronization bit must be included periodically within the information bits, so that the reception of the information can be periodically resynchronized.
In the present invention, the information bits exist for a period of time of 1.3599 MS with a tolerance of 0.027198 MS or 2%. Ideally to insure integrity, the data is to be sampled at the mid point of its period, that is 0.679 MS after it initially appears. The system of the invention described herein requires a 41-bit transmission. Multiplying the number of bits in the transmission 41 times the tolerance per bit 0.027198 MS a timing differential of 1.115 MS can exist. This differential is the accumulated tolerance of a message of 41-bits in length. Since the accumulated error 1.115 MS can exceed the mid point of the period during which data exists, that is 0.679 MS, invalid data can result.
A simple solution to the above mentioned problem would be to send additional synchronization bits, however this approach limits the amount of information data which may be transmitted during a particular time period.
Accordingly, it is an object of the present invention to provide an economical circuit for automatically resynchronizing a lengthy (41-bit) information transmission between a PABX and a central office.
Further, it is an object of the present invention to provide a synchronization circuit which operates without the use of additional resynchronizing bits.